A negative electrode for nonaqueous electrolyte secondary batteries is generally produced by mixing particles of an active material formed from a material into which lithium ions can be inserted by charging, with a binder, a conductive material and a solvent, applying the mixture thus obtained on the surface of a current collector, drying the mixture to form a coating film, and further subjecting the coating film to press processing.
In recent years, along with the development in applications such as electric vehicles and smart phones, there is an increasing demand for capacity increase and lengthening of the service life of batteries. Currently, most of the negative electrodes of commercially available batteries use graphite as the negative electrode active material; however, this active material has already reached the theoretical limit in terms of capacity, and it is now necessary to develop new negative electrode active materials. One of the promising candidates thereof is active materials containing silicon (also referred to as “silicon-based active materials”).
Silicon-based active materials have a potential that the capacity per mass is 5 to 10 times that of graphite. However, on the other hand, silicon-based active materials have a problem that electron conductivity is not so high compared with graphite. Thus, it has been hitherto suggested, in order to increase the electron conductivity of silicon-based active materials, to impart electron conductivity between an additive current collector and the active material by, for example, adding a conductive auxiliary agent.
For example, it has been proposed in Patent Document 1 to attach particles of a metal material having a particle size of 0.0005 μm to 10 μm to the surfaces of silicon-based active material particles.
Furthermore, it has been proposed in Patent Document 2 to coat the periphery of core particles containing silicon with a silicon solid solution such as Mg2Si, CoSi or NiSi, and to further coat the surface with a conductive material such as graphite or acetylene black.
Furthermore, in regard to the silicon-based active materials, suggestions have been made to the effect of enhancing the battery characteristics by controlling the particle size distribution or the particle size.
For example, Patent Document 3 is described, in connection with active material particles containing silicon and/or a silicon alloy, to the effect that when the average particle size of the active material particles is adjusted to from 1 μm to 10 μm, and the particle size distribution is adjusted to a particle size distribution in which 60% by volume or more of the particles have a particle size in the range of from 1 μm to 10 μm, the volume of the active material particles expands and contracts along with the storage and release of lithium resulting from charge and discharge, and thereby an increase in the contact resistance between the active material particles is suppressed.
Patent Document 4 discloses, in connection with a negative electrode active material containing silicon particles, that the active material particles have an average particle size in the range of 7.5 μm to 15 μm, and have a particle size distribution in which 60% by volume or more of the particles have a particle size in the range of average particle size ±40%. It is disclosed to the effect that when the average particle size of the active material particles is adjusted to 7.5 μm or more, the number of particles per volume that exist in the thickness direction of the active material layer becomes smaller, and therefore, the number of particles that should be brought into contact with each other in order to obtain current collectability becomes smaller, so that satisfactory current collectability can be obtained.
Patent Document 5 discloses active material particles containing silicon, which have an average particle size of from 5 μm to 25 μm. When the average particle size of the active material particles is adjusted to 5 μm or more, the original specific surface area of the active material can be reduced. It is described to the effect that since the contact area between the electrolyte and the newly generated surfaces of the active material can be reduced thereby, the effect of enhancing the cycle characteristics and the effect of suppressing swelling of the active material are increased.
Furthermore, in regard to the silicon-based active materials, it has also been suggested to conduct a surface treatment with a silane coupling agent, in order to enhance the cycle characteristics of batteries.
For example, Patent Document 6 describes that, in order to enhance the cycle characteristics of batteries, decomposition caused by the electrolyte at the active material surface is suppressed by surface treating the active material surface with a silane compound such as a silane coupling agent.
Furthermore, Patent Document 7 discloses, in regard to an electrode for lithium ion secondary batteries having a current collector and an active material layer that is formed on the surface of the current collector and contains an active material and a binder polymer, a technology of preventing detachment of the active material caused by expansion and contraction of the electrode at the time of charging and discharging, by chemically bonding the active material and the binder polymer using a silane coupling agent and thereby making the electrode structure strong and stable.